Make pure-python wheels and eggs possible (#467)

setup.py

@@ -88,7 +88,7 @@ def __init__(self, *args, **kwargs):
     macros = [("__LITTLE_ENDIAN__", "1")]
 
 ext_modules = []
-if not PYPY and not PY2:
+if not PYPY and not PY2 and not os.environ.get("MSGPACK_PUREPYTHON"):
     ext_modules.append(
         Extension(
             "msgpack._cmsgpack",